Switch circleci cache key from CIRCLE_SHA1 -> CIRCLE_TAG

[data-sync-user]

Per https://github.com/mozilla-services/Dockerflow/pull/44

The use of CIRCLE_TAG only solves this issue for tagged commits whereas CIRCLE_SHA1 should solve it for dev builds lacking a tag.

See: https://app.circleci.com/pipelines/github/mozilla-services/syncstorage-rs/3955/workflows/90a116aa-9a58-4118-8165-a20c959e1298/jobs/14540

In the “Restoring cache” step you can see that pipeline #3955 job #14540 picks up the cache from the previous pipeline’s (#3952) build-and-test job:

“Found a cache from build 14525 at v1-master-”

┆Issue is synchronized with this Jira Task